An SiO.sub.2 -R.sub.2 O-PbO system or SiO.sub.2 -R.sub.2 O-TiO system silicate glass has heretofore been known as a high refractive index and high optical scattering glass having the optical constants within the above-described ranges as described in, for example, Japanese Patent Publication No. 8394/1967. However, these glasses tend to color and have poor transmission characteristics in the region of from an ultraviolet region to a visible light region, causing disadvantages on the preparation of lenses.
Regarding glass forming materials, to obtain a glass having a high transmittance in the ultraviolet region a phosphate glass is advantageous as compared with a silicate glass or a borate glass for the reasons described below. The phosphate glass has an excellent transmission characteristic in the regions of from an ultraviolet region to a visible light region as compared with that of the silicate glass or borate glass. In addition, the phosphate glass can be glassified by melting at a low temperature, resulting in preventing contamination of impurities from a crucible.
On the other hand, phosphoric acid has a stronger reducing property than other glass forming materials and, as a result, a glass colors depending on the conditions such as components, glass composition, atmosphere, etc. For example, TiO.sub.2, WO.sub.3 and Nb.sub.2 O.sub.5 are conventionally used as the components which give a high refractive index to the glass and improve a chemical durability of the glass and these components cause the source of glass coloration when such are reduced. On the contrary to this, Ta.sub.2 O.sub.5 is not reduced and has less absorption in the regions of from an ultraviolet region to a visible light region. Therefore, Ta.sub.2 O.sub.5 is preferred as a component which gives a high refractive index to a glass and improves the chemical durability of the glass.